SYNTHESIS AND STRUCTURAL EVALUATION OF Mg, Cu, Zn TRI-DOPED LITHIUM TITANIUM OXIDE ELECTRODE MATERIALS FOR USE IN LITHIUM
ION BATTERY
Parbhej Ahamed and Mohammad Abu Yousuf*
Department of Chemistry, Khulna University of Engineering & Technology, Khulna-9203, Bangladesh Received: 25 January 2020 Accepted: 07 March 2020
ABSTRACT
Mg, Cu and Zn metals have been used to tri-dope into the tetrahedral and octahedral sites of Li4Ti5O12 (LTO) electrode materials usually used in Li-ion battery. Li4-xMgxTi5-yZny/2Cuy/2O12 (i. x = 0, y = 0 ii. x = 0.05, y=0.10 iii. x= 0.10, y= 0.20 iv. x= 0.15, y= 0.20) materials were synthesized by solid state reaction using the stoichiometric amount of raw materials. The structural and morphological characteristics of the tri-doped Li4-xMgxTi5-yZny/2Cuy/2O12 materials were methodically analyzed by using Fourier transform infrared spectroscopy (FT-IR), UV-visible spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopic (SEM) techniques. The results of infrared spectroscopy exhibit bands of TO6
octahedra and Ti-O-Ti vibrations. Band gap energy calculation from UV-visible spectroscopy demonstrates a critical point of doping. Beyond this point band gap energy has been found to increase upon tri-doping. It has been observed from SEM images that all samples possess micro-porous and coral shape structures. The XRD patterns demonstrate that Mg, Cu and Zn metals tri-doped Li4-xMgxTi5-yZny/2Cuy/2O12 materials have spinel structure as well as good crystallinity.
Keywords: Li4Ti5O12; Doping; Lithium ion battery 1. INTRODUCTION
Rechargeable lithium-ion batteries (LIBs) are extensively used as electrical energy storage systems and constitute critical elements in the development of sustainable energy technologies. The LIBs are currently being used in airplanes, electric vehicles, small appliances like laptops, mobile phones and in many other places (Scrosati et al., 2010; Tarascon et al., 2009; Yang et al., 2009). They have proved to be a prominent energy storage technology in term of their high energy density, long cycle life, lighter weight and safety (Croguennec et al., 2015). Over the years, the state of the art of the batteries based on the existing anode and cathode materials demand to find advance materials for high performance LIBs. Several electrode materials have been developed since the traditional carbon/graphite anode and LiCoO2 cathode for LIBs (Dell et al., 2000 and Endo et al., 1996) which have been commercialized by Sony group in 1991.It is worthwhile to mention the names ofthe anode materials, such as graphite (Fong et al., 1990), Si (Gómez et al., 2015), Sn (Lübke et al., 2015), Li4Ti5O12
(Yi et al., 2015), metal oxides like SnO2(Li et al., 2015), Co3O4(Wang et al., 2015), NiO(Spinneret al., 2015) used for this purposes.Among them spinel LTO has been found as one of the most promising anode materials for LIBsbecause ofits uniqueproperties. Many factors such as structural stability, longer life cycle, low cost and safety have been considered quite pertinent for use in LIB. It possesses good reversibility with almost negligible volume change during Li-ion insertion/extraction process and provides a good operating voltage at 1.5 V versus lithium. Despite assuming the aforementioned advantages, it exhibits poor electrical conductivity (<10-13 S/cm) (Chen et al., 2001; Guerfi et al., 2004) as compared to the carbon (285 S/cm) (Li et al., 2007). To solve the electrical conductivity limitation of LTO researchers have proposed various ways, such as synthesis of nano- sized LTO (Shen et al., 2003), coating with conductive materials (Yuan et al., 2010) and doping with transition metals (Huang et al., 2004) to shorten the diffusion distance of Li-ion in electrode. Among these methods, the doping offered a direct pathway for changing the material properties of LTO. It has been found that LTO was doped with metal ion such as Mg (Chen et al., 2001), Zn (Yi et al., 2012), Cu (Wang et al., 2013), Al (Zhao et al., 2008), Ta (Hu et al., 2011), V (Yi et al., 2009), Ag (Huang et al., 2004), La (Gao et al., 2010) etc.
Moreover, there have been very few investigations dealing with tri-dopingof LTO structure with metal ions (Shenouda et al., 2008). Accordingly, we have prepared Mg, Zn and Cu metals tri-doped Li4-xMgxTi5-yZny/2
Cuy/2O12 materials by solid state reactions. Structure evaluations of the as prepared Li4-xMgxTi5-yZny/2Cuy/2O12
materials will now be presented here.
2. EXPERIMENTAL 2.1 Sample Preparation
Solid state reaction method was used to prepare Li4-xMgxTi5-yZny/2Cuy/2O12 (i. x = 0, y = 0 ii. x= 0.05, y=0.10 iii.
* Corresponding Author: [email protected] https://www2.kuet.ac.bd/JES/
ISSN 2075-4914 (print); ISSN 2706-6835 (online)
x= 0.10, y= 0.20 iv. X= 0.15, y= 0.20) compounds. LiOH.H2O (Merck, Germany), TiO2 (Merck, Germany), Mg(NO3)2.6H2O (Merck, Germany), Cu(NO3)2.3H2O (Merck, Germany) and Zn(NO3)2.6H2O (Loba Chemie, India) were used without further purification as starting raw materials to prepare Li4-xMgxTi5-yZny/2Cuy/2O12
materials. Stoichiometric quantities of the starting materials were well grounded using agate pestle and mortar for about half an hour to obtaina homogeneous mixture of the raw materials. Then, the mixture was transferred into a crucible and heated in a muffle furnace at 750oC for 6 hours. After calcinations the product Li4-xMgxTi5- yZny/2Cuy/2O12was grounded using agate mortar and kept in desiccators for characterization.
2.2 Fourier Transform Infrared Spectroscopy (FT-IR)
Li4-xMgxTi5-yZny/2Cuy/2O12 materials were well finely grinded with KBr and pressed into pellets using a manual hydraulic pressfor FT-IR measurements. IR Tracer-100 of Shimadzu Corporation, Japan, was used to record the FT-IR spectra. The pellet was scanned in the wave number range of 500-4000 cm-1 with a 2 cm-1 resolution.
2.3 UV-visible Spectroscopy
Appropriate amount of Li4-xMgxTi5-yZny/2Cuy/2O12 samples were suspended inethanol in a cuvette. The cuvette was then placed into its holder for recording UV-visible spectra of the respective sample. UV-1800 spectrophotometer of Shimadzu Corporation, Japan, was used to measure the sample absorption spectra in the range 200-800 nm. Band gap for each of each sample was their respective absorption spectra using Tauc plot.
2.4 Scanning Electron Microscopy (SEM)
Morphology of the prepared Li4-xMgxTi5-yZny/2Cuy/2O12 materials was determined using scanning electron microscopy technique. Prepared samples were suspended in ethanol. Alumina plate was used to adhere conductive carbon tape. 2-3 drops of the suspended sample were added on the surface of the conductive carbon tape. It was then allowed to dry for a sufficient period of time. After drying air blower was used to remove excess powder from the sample plate. The sample plate was finally placed on the chamber for SEM operation.
2.5 Powder X-ray diffraction (XRD)
Bruker Advance D8 XRD diffractometer equipped with a CuKα monochromatic beam (λ = 0.15406Ǻ) was used to identify the phases present in the prepared Li4-xMgxTi5-yZny/2Cuy/2O12 materials. Sample holders of the diffractometer was loaded by pouring with Li4-xMgxTi5-yZny/2Cuy/2O12 materials. It was then mounted on the diffractometer and scanned from 0 to 80o.
2.6 Calculation of x-ray density, bulk density, grain size and porosity
More structural information of the synthesized Li4-xMgxTi5-yZny/2Cuy/2O12 materials was calculated using the following equation.
X-ray density, ρx = (1)
where M is the molecular weight of the corresponding composition, N is the Avogadro’s number (6.023 × 1023),
“a” is the lattice parameter and Z is the number of molecules per unit cell.
Bulk density, ρb = (2)
where m is the mass of the sample, V is the volume of the sample.
Grain size, Dg = . (3)
where, Dg is the crystal size; λ is the wavelength of the X-ray radiation (λ=0.15406 nm) for Cu Kα and is the linewidth at half-maximum height.
Porosity = 1 − × 100 % (4)
The difference between the bulk density, ρb and X-ray density, ρ was considered as a measure of the porosity of the material synthesized.
3. RESULTS AND DISCUSSION
FT-IR spectra of Li4-xMgxTi5-yZny/2Cuy/2O12 materials are shown in Figure 1. The presence of a broad peak in the spectra can be observed at 3428 cm-1. This resulted from the -OH stretching vibrations of the free and hydrogen bonded surface hydroxyl group.The bands near 1650 cm-1 is assigned to the bending stretching vibration of –OH groups (Liu et al., 2006). The absorption bands due to –OH are related to the absorption of water molecules during the preparation of the KBr pellet. Peaks close to 1400 cm-1 are due to the vibrations of CO anion 23 (Zhang et al., 2007). Appearance of peaks at 2850 to 2900 cm-1 is due to the presence of C–H stretching of –
CH2– or –CH3 group. Peaks located near 2330 and 1050 cm-1 are due to the vibration of Ti-O bond and TiO6
octahedra (Zhang et al., 2007 and Li et al., 2008), respectively. UV-visible absorption spectra of the prepared Li4-xMgxTi5-yZny/2Cuy/2O12 materials are presented in Figure 2.
Figure 1: FTIR spectrum of the prepared Li4-xMgxTi5-yZny/2Cuy/2O12 materials
Figure 2: UV-visible spectra of the prepared Li4-xMgxTi5-yZny/2Cuy/2O12 materials (a) Li4Ti5O12, (b) Li3.95Mg0.05Ti4.90Cu0.05Zn0.05O12, (c) Li3.90Mg0.10Ti4.80Cu0.10Zn0.10O12 and (d) Li3.85Mg0.15Ti4.80Zn0.10Cu0.10O12
Table 1: Band gap energy Li4-xMgxTi5-yZny/2Cuy/2O12 anode materials
Sample Band gap energy, E (eV)
Li4Ti5O12(LTO) 5.6292
Li3.95Mg0.05Ti4.90Cu0.05Zn0.05O12(LTOM1) 5.5344 Li3.90Mg0.10Ti4.80Cu0.10Zn0.10O12 (LTOM2) 5.4954 Li3.85Mg0.15Ti4.80Zn0.10Cu0.10O12 (LTOM3) 5.7504
Band gap energy of Li4-xMgxTi5-yZny/2Cuy/2O12 materials was calculated by extrapolation of the Tauc plot. The calculated band gap energies are summarized in the following Table 1. Tri-doping of Li4-xMgxTi5-yZny/2Cuy/2O12
materials has decreased the band gap energies except Li3.85Mg0.15Ti4.80 Zn0.10Cu0.10O12. It is obvious that
lowering of band gaps of tri-doped Li4-xMgxTi5-yZny/2Cuy/2O12 materials have resulted from the larger radius of the dopant ions (Mg2+ = 86 pm, Zn2+ = 88 pm, Cu2+ = 87 pm) compared to the host ions (Li+ = 90 pm, Ti4+ = 74.5 pm). In the Li4Ti5O12 compound, the valence band (VB) and the conduction band (CB) consist of Li 2s, Ti 3d and O 2p orbitals. When Li4Ti5O12 compound is tri-doped with Mg, Cu, Zn metals, and the valence state of tri-doped Li4-xMgxTi5-yZny/2Cuy/2O12 compound subsequently become more delocalized with Mg 2p, Cu 3d and Zn 3d orbital’sresulting mixing of Mg 2p, Cu 3d and Zn 3d state with VB, and thus increases the width of the VB itself. A plot of band gap energy with doping in tri-doped Li4-xMgxTi5-yZny/2Cuy/2O12 is presented in Figure 3. It is obvious that the band gap energy of the tri-doped Li4-xMgxTi5-yZny/2Cuy/2O12 materials has decreased up to a certain amount of dopants.
Figure 3: Variation of band gap energy in Li4-xMgxTi5-yZny/2Cuy/2O12
(a) Li4Ti5O12 (b)Li3.95Mg0.05Ti4.90Zn0.05Cu0.05O12
(c) Li3.90Mg0.10Ti4.80Zn0.10Cu0.10O12 (d) Li3.85Mg0.15Ti4.80Zn0.10Cu0.10O12
Figure 4: SEM images of the prepared sample Li4-xMgxTi5-yZny/2Cuy/2O12.
Then it increased with increasing the amount of doping. This behavior is expected because of the incorporation of dopant into the crystal lattice. With increasing the amount of dopant some of them could not completely incorporate into the spinel structure of Li4Ti5O12. As a result, the interaction between the cations and anions might have been changed. This ultimately affects the electronic state of the valence band of Li4-xMgxTi5-yZny/2
Cuy/2O12 materials. Eventually it generates the anomalies in the band gap of Mg, Cu, and Zn tri-doped of Li4- xMgxTi5-yZny/2Cuy/2O12 compounds.
The surface morphology of the prepared Li4-xMgxTi5-yZny/2Cuy/2O12 materials was investigated by scanning electron microscopy (SEM). Figure 4 shows the representative SEM images of the prepared materials. It is
interesting to note that the morphology of the tri-doped samples has hardly changed. Morphology of the tri- doped sample is very similar to the un-doped samples. Based on this observation it is concluded that metals are homogeneously tri-doped into Li4-xMgxTi5-yZny/2Cuy/2O12. All the prepared samples exhibit micro-porous and coral shape structures. This coral shape structure is associated with sufficient pores. Each of these pores may offer short pathways for Li-ion diffusion. Furthermore, the well decorated pores of the coral structures may act as active sites and provide more ion channels for extraction or insertion processes of Li-ion into Li4-xMgxTi5- yZny/2Cuy/2O12 materials. Thus, each of these Li4-xMgxTi5-yZny/2Cuy/2O12 materials having sufficient active sites and diffusion channels may be used as electrode materials in Li-ion battery.
Figure 5 shows the representative XRD patterns of the prepared Li4-xMgxTi5-yZny/2Cuy/2O12 samples. All the diffraction peaks can be well indexed to a cubic spinel structure of Li4Ti5O12 (JCPDS card No. 00-049-0207).
No peaks from impurities are observed. This indicates that the metals have been successfully entered into the lattice of Li4-xMgxTi5-yZny/2Cuy/2O12.
Figure 5: XRD spectra of the prepared samples Li4-xMgxTi5-yZny/2Cuy/2O12
Diffraction peaks of the prepared Li4-xMgxTi5-yZny/2Cuy/2O12 samples can be indexed as (111), (220), (311), (400), (331), (333), (440) and (531) of the cubic Li4Ti5O12 (Guo et al., 2015). This confirmed that tri-doping do not change the basic structure of Li4Ti5O12. Then the aforementioned indexed planes were used to calculate interplanar spacing, lattice constant and grain sizes of the prepared Li4-xMgxTi5-yZny/2Cuy/2O12 samples. The calculated structural parameters from the analysis of the XRD patterns are summarized in the following Table 2.
The lattice parameters of Li4-xMgxTi5-yZny/2Cuy/2O12 materials are calculated through the Bragg’s equation from the diffraction peaks of (111), (220), (311), (400), (331), (333), (440) and (531) planes. The average lattice parameters obtained by analysis of the X-ray pattern werefound to be 8.3545Ǻ for Li4Ti5O12, 8.3597 Ǻ for Li3.95Mg0.05Ti4.90Zn0.05Cu0.05O12, 8.3585Ǻ for Li3.90Mg0.10Ti4.80Zn0.10Cu0.10O12, 8.3427 Ǻ for Li3.85Mg0.15Ti4.80
Zn0.10Cu0.10O12. Moreover, the variation in the calculated lattice parameter and unit cell volume of the tri-doped Li4-xMgxTi5-yZny/2Cuy/2O12 materials are presented in Figure 6.
Lattice parameters for Li3.95Mg0.05Ti4.90Zn0.05Cu0.05O12 and Li3.90Mg0.10Ti4.80Zn0.10Cu0.10O12 composition has been found to increase than that of Li4Ti5O12. This increase of lattice parameter resulted from the larger radius of the dopant ions (Mg2+ = 86 pm, Zn2+ = 88 pm, Cu2+ = 87 pm) compared to the host ions (Li+ = 90 pm, Ti4+ = 74.5 pm) (Lu et al., 2016). The unit cell volume of the tri-doped Li4-xMgxTi5-yZny/2Cuy/2O12 materials has been distorted upon changing the dopant amount. Mismatch of the unit cell volume due to doping can be represented as shown in the following Figure 6.
More structural information related to the crystal structure of Li4-xMgxTi5-yZny/2Cuy/2O12 materials has been calculated using equation (1), (2) (3) and (4). The values of X-ray density, bulk density and percentage of porosity obtained using equation (1), (2) and (4), respectively are listed in Table 3. X-ray density ρ of the tri- doped Li4-xMgxTi5-yZny/2Cuy/2O12 increased with increasing dopant concentrations. The substitution of low
Position [°2Theta] (Copper (Cu))
10 20 30 40 50 60 70
Counts/s
0 500 1000
P1- 18.03.18_LMgTCZ
Position [°2Theta] (Copper (Cu))
10 20 30 40 50 60 70
Counts/s
0 500 1000
P3- 19.03.18_LMgTCZ
Position [°2Theta] (Copper (Cu))
10 20 30 40 50 60 70
Counts/s
0 500 1000
P5- 19.03.18_LMgTCZ
Position [°2Theta] (Copper (Cu))
10 20 30 40 50 60 70
Counts/s
0 500 1000
P2- 18.03.18_LMgTCZ
(a) Li4Ti5O12
(d) Li3.85Mg0.15Ti4.80Zn0.10Cu0.10O12
(c) Li3.90Mg0.10Ti4.80Zn0.10Cu0.10O12
(b) Li3.95Mg0.05Ti4.90Zn0.05Cu0.05O12
atomic mass Li and Ti with large atomic mass Mg, Cu and Zn in accordance with the enhancement of X-ray density of tri-doped Li4-xMgxTi5-yZny/2Cuy/2O12. In contrast, the calculated values of bulk density of tri-doped Li4-xMgxTi5-yZny/2Cuy/2O12 decreased with increasing the concentration of dopants. Results of the X-ray density and bulk density have been used to calculate the percentage of porosity present in the tri-doped Li4-xMgxTi5- yZny/2Cuy/2O12. It can be seen from Table 3 that the percentage of porosity increased with increasing the dopant concentration. The variation of density with Mg, Cu and Zn content in Li4-xMgxTi5-yZny/2Cuy/2O12 are depicted in Figure 7(a). Variation in the percentage of porosity present in Li4-xMgxTi5-yZny/2Cuy/2O12 materials is also presented in the Figure 7(b).
Table 2: Position of the X-ray peaks, interplanar spacing, corresponding miller indices and grainsize of each plane
Li4Ti5O12
2θ 18.345
6 30.183
6 35.578
1 43.266
1 47.417
3 57.348
4 63.097
4 66.376 d (Ǻ) 4.8361 2.9609 2.5234 2.0911 1.9173 1.6053 1.4722 1.4072 2 (hkl) (111) (220) (311) (400) (331) (333) (440) (531)
a 8.3763 8.3746 8.3691 8.3644 8.3573 8.3413 8.3280 8.3251
aavg(Ǻ) 8.3545
V (Ǻ)3 583.12
Dg (nm) 68 34 105 108 55 188 97 65
Dg, avg (nm) 90
Li3.95Mg0.05Ti4.90
Zn0.05Cu0.05O12
2θ 18.357
4 30.241
9 35.573
2 43.237
3 47.381
0 57.247
4 62.886
9 66.142 d (Ǻ) 4.8330 2.9554 2.5237 2.0907 1.9171 1.6079 1.4766 1.4116 9 (hkl)) (111) (220) (311) (400) (331) (333) (440) (531)
a 8.3710 8.3591 8.3701 8.3628 8.3564 8.3548 8.3529 8.3511
aavg(Ǻ) 8.3597
V (Ǻ)3 584.21
Dg (nm) 81 41 60 88 60 94 97 131
Dg, avg (nm) 82
Li3.90Mg0.10Ti4.80
Zn0.10Cu0.10O12
2θ 18.392
5 30.237
7 35.593
7 43.264
9 47.389
8 57.263
8 62.909
5 66.150 d (Ǻ) 4.8238 2.9558 2.5223 2.0912 1.9183 1.6088 1.4764 1.4114 1 (hkl) (111) (220) (311) (400) (331) (333) (440) (531)
a 8.3550 8.3602 8.3655 8.3648 8.3616 8.3595 8.3517 8.3499
aavg(Ǻ) 8.3585
V (Ǻ)3 583.96
Dg (nm) 68 69 84 108 36 91 97 78
Dg, avg (nm) 79
Li3.85Mg0.15Ti4.80
Zn0.10Cu0.10O12
2θ 18.389
4 30.245
9 35.617
4 43.314
0 47.457
2 57.428
7 63.185
8 66.488 d (Ǻ) 4.8246 2.9550 2.5207 2.0889 1.9158 1.6033 1.4703 1.4051 6 (hkl) (111) (220) (311) (400) (331) (333) (440) (531) a (Ǻ) 8.3564 8.3580 8.3602 8.3556 8.3507 8.3309 8.3172 8.3126
aavg(Ǻ) 8.3427
V (Ǻ)3 580.65
Dg (nm) 81 69 105 86 73 75 97 131
Dg, avg (nm) 90
(a) LTO (b) LTOM1 (c) LTOM2 (d) LTOM3
Figure 6: Mismatch of cubic unit cell of Li4-xMgxTi5-yZny/2Cuy/2O12due to doping 8.3545 a =
Ao
a=
8.3597 Ao
a = 8.3585 Ao
8.3427 a = Ao
Table 3: X-ray density, bulk density and porosity of the prepared samples
Sample X-ray density,
ρx (g/cm3) Bulk density,
ρb (g/cm3) Porosity (%)
Li4Ti5O12 10.080 1.010 89.98
Li3.95Mg0.05Ti4.90Zn0.05Cu0.05O12 10.112 0.976 90.64 Li3.90Mg0.10Ti4.80Zn0.10Cu0.10O12 10.576 0.954 90.97 Li3.85Mg0.15Ti4.80Zn0.10Cu0.10O12 10.617 0.944 91.10
Figure 7: Variation of (a) bulk density and X-ray density and (b) bulk density and porosity of the tri-dope Li4-x
MgxTi5-yZny/2Cuy/2O12 materials 4. CONCLUSION
Micro-porous and coral shape Li4-xMgxTi5-yZny/2Cuy/2O12 (i. x = 0, y = 0 ii. x= 0.05, y=0.10 iii. x= 0.10, y= 0.20 iv. X=
0.15, y= 0.20) compounds were prepared by solid state reaction method. XRD analysis revealed that Mg, Cu and Zn were tri-doped into Li4-xMgxTi5-yZny/2Cuy/2O12 structure. The tri-doping of Mg, Cu and Zn metals into Li4-xMgxTi5- yZny/2Cuy/2O12 structure depends upon a certain value of x and y and beyond that value inhibit complete incorporation into Li4Ti5O12 structure. Minimum band gap energy of Mg, Cu and Zn tri-doped Li4-xMgxTi5-yZny/2Cuy/2O12 resulted from the saturated amount of dopants. Band gap energy of Mg, Cu and Zn tri-doped Li4-xMgxTi5-yZny/2Cuy/2O12 are found to decrease to certain amount of the dopant. Then it increased with increasing the amount of dopant. Li4Ti5O12
as well as Mg, Cu and Zn tri-doped Li4-xMgxTi5-yZny/2Cuy/2O12 shows micro-porous and coral shape morphology.
Diffraction peaks from XRD spectra represented cubic spinel structure of Li4-xMgxTi5-yZny/2Cuy/2O12 materials. A linear variation of X-ray density, bulk density and percentage of porosity with increasing the amount of dopant has been observed in the tri-doped Li4-xMgxTi5-yZny/2Cuy/2O12 materials.
ACKNOWLEDGEMENT
The authors are thankful to the Committee for Advanced Studies and Research of Khulna University of Engineering & Technology for providing fund of this research.
CONFLICT OF INTERESTS
The authors declare that they have no conflict of interests.
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